Remote sensing of cloud droplet radius profiles using solar reflectance from cloud sides – Part 1: Retrieval development and characterization

Abstract

Abstract. Convective clouds play an essential role for Earth's climate as well as for regional weather events since they have a large influence on the radiation budget and the water cycle. In particular, cloud albedo and the formation of precipitation are influenced by aerosol particles within clouds. In order to improve the understanding of processes from aerosol activation, from cloud droplet growth to changes in cloud radiative properties, remote sensing techniques become more and more important. While passive retrievals for spaceborne observations have become sophisticated and commonplace for inferring cloud optical thickness and droplet size from cloud tops, profiles of droplet size have remained largely uncharted territory for passive remote sensing. In principle they could be derived from observations of cloud sides, but faced with the small-scale heterogeneity of cloud sides, “classical” passive remote sensing techniques are rendered inappropriate. In this work the feasibility is demonstrated to gain new insights into the vertical evolution of cloud droplet effective radius by using reflected solar radiation from cloud sides. Central aspect of this work on its path to a working cloud side retrieval is the analysis of the impact unknown cloud surface geometry has on effective radius retrievals. This study examines the sensitivity of reflected solar radiation to cloud droplet size, using extensive 3-D radiative transfer calculations on the basis of realistic droplet size resolving cloud simulations. Furthermore, it explores a further technique to resolve ambiguities caused by illumination and cloud geometry by considering the surroundings of each pixel. Based on these findings, a statistical approach is used to provide an effective radius retrieval. This statistical effective radius retrieval is focused on the liquid part of convective water clouds, e.g., cumulus mediocris, cumulus congestus, and trade-wind cumulus, which exhibit well-developed cloud sides. Finally, the developed retrieval is tested using known and unknown cloud side scenes to analyze its performance.

Multi-View Polarimetric Scattering Cloud Tomography and Retrieval of Droplet Size

“…Operational remote sensing algorithms, which rely on 1D RT and plane-parallel cloud models, retrieve a single value for r e (and for v e ), for each cloudy pixel. This occurs both in bi-spectral [9,83] and polarimetric [11,84] techniques. The physical interpretation is complicated by the difficulty in determining which portion of the cloud the retrieved quantities corresponds to.…”

Why we need radar, lidar, and solar radiance observations to constrain ice cloud microphysics

“…It measures spectral radiance with a spectral resolution of 3 nm in the visible and 10 nm in the infra-red. As a push broom scanner, its spatial resolution is in the order of 10 m for cloud objects at a distance of about 10 km The system is well characterized and calibrated (Ewald et al, 2015), while first retrievals for cloud optical properties were developed (Zinner et al, 2016;Ewald et al, 2019b).…”

Retrieval of the vertical evolution of the cloud effective radius from the Chinese FY-4 (Feng Yun 4) next-generation geostationary satellites

“…The cloud effective radius (R e ) is the core parameter representing the microphysical characteristics of clouds and is closely related to the processes forming precipitation. Freud and Rosenfeld (2012) showed that the rate of droplet coalescence is proportional to the fifth power of the mean volume radius, which means that the change in the droplet coalescence rate is small when R e is small and warm rain is ef-ficiently formed when R e is > 14 µm. Similarly, for marine stratocumulus clouds, when R e is < 14 µm, the column maximum rain intensity is almost < 0.1 mm h −1 , but the intensity of rain increases rapidly as R e exceeds this threshold, regardless of the cloud water path (Rosenfeld et al, 2012a).…”